67 Pro Reverb - Ch 2 noise

[Randall]

Glad you got it dialed in and replaced those carbon comp resistors. Funny how some swear they are part of the mojo of that era amp, yet when you place the old tired and cranky ones with good modern ones, the amp comes alive again. Blackface Pro Reverbs are unbeatable IMHO.

[fdesalvo]

Preach it, man. I’m beginning to believe the transformers and drifted resistors are the keys to the magic.

[Randall]

Transformers maybe, drifted resistors? Nah. Put it back to how Leo designed it and call it good IMHO. Lots and lots of debate on this, but that's where I live, and no good evidence to the contrary.

[Justin Thomas]

RG Keen has a very nice write-up on CC resistor mojo. But the sumary is that they would only make a positive difference in one or two positions, somewhere in the phase inverter. But he actually measures and is an engineer, so I know he's not just spouting BS. It's on geofex.com.

Justin

[Justin Thomas]

I’m beginning to believe ... drifted resistors are the keys to the magic.

While I do believe that a drifted resistor can have an effect on tone, if I get one drifted out of its actual tolerance (which in all my amps happens to be 10%) then I will remove it, measure it, & replace it with a new resistor of the measured value. Same with caps. I won't leave a failed part in, but if the drifted value won't hurt the amp, switcheroo.

Value matters a LOT more than composition.

Justin

[fdesalvo]

Ya I can see that.

Also, value over comp for certain. I used what I had on hand, so there are carbon/metal films and metal oxides present. Sounds stunning.

[Chuck H]

Since we're talking about the old Fender amp mojo... It's been noted many times that certain specimens can sound amazing while others just sound ordinary. Of course "ordinary" for these amps is very good, so that wasn't to say the ordinary amps lack anything. But then there's those special ones. Hmmm.?.

I think that some of that phenomenon can be attributed to the wide spec tolerance of the old resistors. Maybe some more to drifted resistors. Maybe a little to the affect of minor board conductivity, which happens but is oft overlooked. And I recently noticed another possible contributor. Most players prefer the vibrato/reverb channel on these amps even if they don't use either effect. The standard lore is that it's because that channel has a tad more gain. But something else is happening there too. There are like phase gain stages sharing a filter node. If the filter there has a little extra impedance due to age, disuse or 'just because it does' then the opportunity for a small amount of positive feedback exists. And this condition would be different for every amp so it is another variable. Dumble has been known to incorporate very low level positive feedback loops in some amps. Another Hmmm.?. And Fender, possibly in their wisdom, uses a similar power supply arrangement for their most popular later series amps. The HR's which are known to squeal when the filters are even a little off spec. Another Hmmm.?. So I've thrown this new element onto the pile of possibilities why some amps are more outstanding.

[fdesalvo]

I like it, Chuck.

I’ve played about 10 or so vintage fender amps that weren’t ones I’ve done electronic restoration on. I’ve worked on maybe a dozen. Brought them within 10% of spec in most places and have liked the tone of all of them. I’ve truly only loved the tone of maybe 3 and this is one of them. Took me so long to figure out what I wanted to hear and this is it. I’m def a 12” driver guy and love the big bottles, though I’m running some Sylvania 6l6wgb’s here.

[Justin Thomas]

Given that tolerance of +/-20% can add up to a possible 40%variance between 2 different parts...

I have definitely heard sone dog vibtage amps, but I have never seen one that didn't have a LOT of raw potential without requiring an extensive rework. Everything from LectroLab to Vibro-King. The LectroLab became a screamer with a good speaker and a single cathode bypass cap added. Of course I told the owner to buy an extension cab, because that cardboard baffle wasn't gonna hold that Celestion for long...

Routine maintenance and a good work-through has rendered all but one a great machine. They may not be to MY tastes necessarily, but they all turned out maiing someone happy to play them.

Fender just seems to be the best candidate for "raw stock" - great trannies, a solid chassis, lots of holes already drilled, options! There are many others too, though.

Incidentally the worst Fender (vintage) I ever heard was a BF Bassman. I don't know which circuit it was, but it was stringy, weak, and had no guts. I wasn't about to shell out $700 for it when my own BF Bassman spanked it, but I woulda gave it a once-over cuz I'm sure there was a good reason it sucked...

Justin

[g1]

love the big bottles, though I’m running some Sylvania 6l6wgb’s here.

Are those the JAN versions from the '80s ? If so, I'd call them big bottle tubes in a compact package.

[fdesalvo]

Ya the ol mil servo tubes iirc.